Sulphonated polyimides, membranes prepared with them, and a fuel cell device that includes these membranes

ABSTRACT

The invention relates to sulphonated polyimides, notably of formula (I)  
                 
 
     The invention also relates to an ion exchange membrane that includes such a polyimide and a fuel cell that includes such a membrane.  
     The membranes of the invention have excellent durability and low cost and the fuel cells can be used, in particular, in electric vehicles.

[0001] This invention relates to sulphonated polyimides, which findapplication particularly in the preparation of ion exchange membranesnotably for the manufacture of fuel cells.

[0002] The use of solid polymer electrolytes was proposed in the 1950sand applied notably in the construction of fuel cells which wereintended particularly to supply space craft with energy.

[0003] The interest in fuel cells is now progressing beyond thegeneration of power for space craft and the automobile industry hasinterest in them for at least two reasons.

[0004] the first rests on the concern to avoid pollution caused byinternal combustion engines. In effect it is clear that it will bedifficult to prevent all discharges of nitrogen oxides, unburnthydrocarbons and oxygenated compounds by means of all the improvementsthat one can expect through better control of combustion.

[0005] the second reason, for the longer term, is to research motorsthat use a fuel other than the fossil fuels that it is known will notlast for ever.

[0006] Any system based on hydrogen can respond to the concernsmentioned above. The source of supply is potentially inexhaustible andelectrochemical combustion only produces water.

[0007] The schematic assembly of a fuel cell that permits at the sametime the production of electrical energy and incidentally the synthesisof water for the needs of the crew of a space vehicle, is represented inpart in FIG. 1 appended.

[0008] The ion exchange type of membrane formed from a solid polymerelectrolyte (1), is used to separate the anode compartment (2) whereoxidation occurs of the fuel, such as hydrogen H₂ (4) according to theequation:

2H₂→4H⁺+4e⁻;

[0009] from the cathode compartment (3) where the oxidant such as oxygenO₂ is reduced according to the equation:

O₂+4H⁺+4e⁻→2H₂O

[0010] with production of water (6) while the anode and the cathode areconnected through an external circuit (10).

[0011] The anode (7) and the cathode (8) are essentially constituted bya porous support, for example made of carbon, on which particles of anoble metal such as platinum are deposited.

[0012] The membrane and electrode assembly is a very thin assembly witha thickness of the order of a millimeter and each electrode is suppliedfrom the rear with the gases using a fluted plate.

[0013] One very important point is to properly maintain the membrane inan optimum moisturised state so as to ensure maximum conductivity.

[0014] The membrane has a double role. On the one hand it acts as anionic polymer permitting the transfer (9) of hydrated protons H₃O⁺ fromthe anode to the cathode, and on the other hand it keeps each of thegases oxygen and hydrogen in their compartments.

[0015] The polymer constituting the membrane must therefore fulfil acertain number of conditions relating to its mechanical,physico-chemical and electrical properties.

[0016] First of all, the polymer must be able to give thin films,between 50 and 100 micrometers thick, which are dense and withoutdefects. The mechanical properties, rupture stress modulus, ductility,must make it compatible with the assembly operations which include, forexample, being clamped between metal frames.

[0017] The properties must be conserved when it passes from a dry to amoist state.

[0018] The polymer must have good thermal stability to hydrolysis andexhibit good resistance to reduction and oxidation up to 100° C. Thisstability shows itself in terms of variation in ionic resistance and interms of variation in mechanical properties.

[0019] Finally, the polymer must have high ionic conductivity, thisconductivity is provided by strongly acidic groups such as phosphoricacid groups, but above all by sulphonic groups linked to the polymerchain. Because of this these polymers will generally be specified bytheir equivalent mass, that is to say, the weight of polymer in gramsper acid equivalent.

[0020] By way of example, the best systems developed at present arecapable of supplying a specific power of 1 W.cm⁻², or a current densityof 4 A.cm⁻² for 0.5 Volts.

[0021] Since 1950, numerous families of polymers or sulphonatedpolycondensates have been tested as membranes without it being atpresent possible to establish with certainty the relationships betweenchemical structure, film morphology and performance.

[0022] At first, sulphonated phenolic type resins prepared bysulphonation of polycondensed products such as phenol-formaldehyderesins were used.

[0023] The membranes prepared with these products are low cost, but theydo not have sufficient stability to hydrogen at 50-60° C. forapplications of long duration.

[0024] Next one turned towards sulphonated polystyrene derivatives whichhave greater stability compared with that of the sulphonated phenolicresins but cannot be used at more than 50-60° C.

[0025] At the present time, the best results are obtained withcopolymers, the linear main chain of which is perfluorinated and theside chain of which carries a sulphonic acid group.

[0026] These copolymers are commercially available under the trademarkNAFION® from the Du Pont Company or ACIPLEX-S® from the Asahi ChemicalCompany. Others are experimental, products by the DOW Company for themanufacture of the membrane named “XUS”.

[0027] These products have been the subject of numerous developments andconserve their properties for several thousands of hours between 80 and100° C. with current densities that depend on the partial pressures ofthe gases and the temperature. The current density is typically 1 A.cm⁻²at 0.7 Volts for Nafion® 112 with a thickness of 50 μm.

[0028] The polymers of the Nafion® type are obtained byco-polymerisation of two fluorinated monomers, one of which carries theSO₃H group. A second route for obtaining perfluorinated membranes hasbeen explored in documents by G. G. Scherer: Chimia, 48 (1994), p.127-137; and by T. Monose et al., patent U.S. Pat. No. 4,605,685. Itinvolves the grafting of styrene or fluorinated styrene monomers ontofluorinated polymers which are subsequently sulphonated. These membraneshowever have properties close to those of fluorinated co-polymers.

[0029] If one tries to draw lessons from the teachings of the prior art,it is apparent that the best chemical structure for a polymer that canbe used in the form of a membrane for the exchange of protonscorresponds to the following criteria:

[0030] a main chain totally perfluorinated

[0031] branches bearing a sulphonic acid group

[0032] equivalent weight between 800 and 1200.

[0033] In the documents by W. Grot; Chem. Ing. Tech., 50, 299 (1978) andby G. G. Scherer: Phys. Chem., 94, 1008-1024 (1990) they claim for thesestructures “very good thermal stabilities”; however, it should be takeninto consideration that the notion of thermal stability has to be takenhere as the ability to resist acid hydrolysis at a temperature between60 and 100° C. over a period of several thousands of hours and thattherefore the information from these documents must be consideredprudently.

[0034] To that, it would be proper to add resistance to oxidation incontact with oxygen in the cathode compartment and resistance toreduction in the presence of H₂.

[0035] On the other hand, from the viewpoint of the development of fuelcells that can be used for automobile traction, another importantproblem that will henceforth be clearly identified by the experts is thecost of the membrane.

[0036] In 1995, the cost of membranes produced or under development wasof the order of 3000 to 3500 French francs per square meter and onemight estimate that it would be necessary to divide this cost by 10 orindeed 20 in order for it to play a part in the industrial developmentof fuel cells for the automobile industry.

[0037] With a view to lowering the costs, poly 1,4-(diphenyl-2,6)-phenylethers, sulphonated on the main chain, the polyether-sulphones andpolyether-ketones have been synthesised and tested without reallyholding their own against the fluorinated membranes with regard to theirimmediate performance and their durability.

[0038] In effect, the rigidity of the chains makes these productsinsoluble and it becomes difficult to obtain the thin films necessaryfor the creation of the membranes.

[0039] There therefore exists an unsatisfied need for spolymers whichcan be easily made into the form of membranes, namely of thin filmswhich meet the conditions already mentioned above relating notably totheir mechanical, physico-chemical and electrical properties, inparticular those relating to their thermal stability and theirresistance to acid hydrolysis, at elevated temperature for a long periodof time, their resistance to oxidation in contact with oxygen as well astheir resistance to reduction in the presence of hydrogen.

[0040] Furthermore, there exists a need for membranes which, at the sametime as satisfying the properties above, can be manufactured at lowcost, by a simple method with raw materials that are readily available.

[0041] The aim of this invention is to provide a polymer which satisfiesthe group of needs previously mentioned.

[0042] A further aim of the invention is to provide membranes thatinclude or are prepared with this polymer and a fuel cell that includesthese membranes.

[0043] These aims and others are met conforming to the invention by apolyimide that comprises repeating structures of formula (I_(n)).

[0044] and repeating structures of formula (I_(m))

[0045] in which

[0046] the groups C₁ and C₂ can be identical or different and eachrepresent a tetravalent group that includes at least one aromatic carbonring possibly substituted and having from 6 to 10 atoms of carbon and/ora heterocyclic ring with aromatic character, possibly substituted andhaving from 5 to 10 atoms and including one or more heteroatoms chosenfrom among S, N, and C; C₁ and C₂ each forming with the neighbouringimide groups rings with 5 or 6 atoms.

[0047] The Ar₁ and Ar₂ groups can be identical or different and eachrepresent a divalent groups that includes at least one aromatic carbonring possibly substituted and having from 6 to 10 atoms of carbon and/ora heterocyclic ring with aromatic character, possibly substituted andhaving from 5 to 10 atoms and including one or more heteroatoms chosenfrom among S, N and O; at least one of said aromatic carbon rings and/orheterocyclic rings of Ar₂ being, in addition substituted by at least onesulphonic acid group.

[0048] The repeating structure (In) is repeated k times and therepeating structure (Im) is repeated k times, j and k being two wholenumbers.

[0049] Preferably, j represents a whole number from 1 to 200, morepreferably from 4 to 60 and k represents a whole number from 1 to 300,preferably from 4 to 120.

[0050] The co-polymer according to the invention, depending on thepositioning of the two structures which make it up, can be defined asbeing a sequential, alternating or a statistical co-polymer.

[0051] However the polyimide according to the invention which can bedefined as a sulphonated polyimide corresponds preferably to thefollowing general formula (I):

[0052] in which C₁, C₂, Ar₁ and Ar₂ have the meanings already given tothem above and where each of the groups R₁ and R₂ represent NH₂ or agroup of formula

[0053] where C₃ is a divalent group that includes at least one aromaticcarbon ring possibly substituted and having from 6 to 10 carbon atomsand/or a heterocyclic ring with aromatic character, possibly substitutedand having from 5 to 10 atoms and including one or more heteroatomschosen from among S, N and O.

[0054] C₃ forming with the neighbouring imide group a ring with 5 or 6atoms.

[0055] In the formula (I) above

[0056] m represents a whole number preferably from 1 to 20, morepreferably from 2 to 10;

[0057] n represents a whole number preferably from 1 to 30, morepreferably from 2 to 20;

[0058] o represents a whole number preferably from 1 to 10, morepreferably from 2 to 6;

[0059] The molecular weight of the polyimide according to the inventionis generally from 10,000 to 100,000, preferably from 20,000 to 80,000.

[0060] The equivalent molecular weight of the polyimide according to theinvention is preferably from 400 to 2500, more preferably from 500 to1200.

[0061] Because of this, the numbers m and n (j and k) will be chosen insuch a way that the equivalent molecular weight shall be from 400 to2500, preferably from 500 to 1200, the equivalent molecular weighthaving been defined above.

[0062] In a general way, it is known that the heterocyclic polymers andin particular the polyimides can allow one to obtain films thanks totheir synthesis in two steps.

[0063] These “heterocyclic” polymers are used, for example inaeronautical and space applications which require excellent mechanicalproperties and good resistance to oxidation. These applications are verymuch removed from the field of this patent application.

[0064] The specific sulphonic co-polyimides of this invention offer, ina surprising manner, all the properties already mentioned above as beingrequired for the production of membranes, in particular cation exchangemembranes, notably for fuel cells, the performances of which arecompatible with the envisaged applications.

[0065] In particular, The specific co-polymers according to theinvention can be easily formed into films or membranes of a suitablethickness.

[0066] The polymers according to the invention have a very high ionexchange capacity greater than 0.4 meq/g, for example from 0.8 to 2.5meq/g, which is greater than the ion exchange capacity of the polymersof the prior art which generally achieve only a maximum of 0.9 to 1.2meq/g.

[0067] The membranes comprising the polymers according to the inventionalso have high thermal stability, for example to acid hydrolysis at hightemperature, that is to say for the most stable membranes up totemperatures that can reach, for example 100° C., and this for a longduration that can extend, for example, to 5000 hours.

[0068] These conditions are the conditions of use that can prevail inthe cells where the membranes are put to use.

[0069] Similarly, the membranes according to the invention haveexcellent resistance to reduction and to oxidation.

[0070] The invention is therefore totally dissociated from the prior artmentioned above in which the polymers recommended for the manufacture ofmembranes for the exchange of cations and in particular protons, notablyfor fuel cells have a structure fundamentally different from that of thepolymers of the polyimide type of the present application for a patent.

[0071] This patent application departs radically and in a surprising wayfrom the processes of the prior art by preparing specific polyimideswith a view to their use in cation exchange membranes.

[0072] In effect, in a general way, the polyimides have been neithermentioned not proposed for use in this field, on the other hand, thespecific polyimides according to the invention have mechanical,physico-chemical and electrical properties superior to those of thepolymers of the prior art as is demonstrated below.

[0073] Nothing allowed one to suppose that the polyimides according tothe invention were going to totally satisfy the requirements expressedand until now not satisfied for the preparation of cation exchangemembranes.

[0074] Finally, as described below, the polyimides according to theinvention are prepared in a simple manner, by methods proven on theindustrial scale and from raw materials that are available and which arelow cost. Because of this, the membranes obtained and the fuel cellsthat include these membranes will also see their costs much reduced.

[0075] The invention will now be described in more detail, makingreference to the attached drawings in which:

[0076]FIG. 1 diagrammatically represents a fuel cell and itselectrode-membrane-electrode assembly.

[0077] The membrane can notably be a membrane that comprises a polymeraccording to this invention.

[0078]FIG. 2 shows a graph with on the y-axis, the voltage expressed inmV and on the x-axis the current density expressed in A/cm² on which areplotted the polarisation curves obtained with non-bonded “E-TEK”electrodes (0.35 mg of Pt/cm² and 0.8 mg of Nafion®/cm²) andrespectively a Nafion®117 membrane (curve drawn with the broken line)and a phthalic polyimide membrane of this invention (curve drawn withthe full line). The operating temperature of the cell was 50° C. and thepressures of H₂ and O₂ were 4 bars.

[0079]FIG. 3 is a graph analogous to that of FIG. 2 but the polarisationcurves are those obtained on the one hand with a Nafion®117 membrane(curve drawn a with broken line) and on the other hand with anaphthalenic polyimide of this invention (curve drawn with a full line)and the operating temperature of the cell was 70° C.

[0080] In the formulae (In), (Im) and (I) cited above, C₁ and C₂ can beidentical or different and each represents, for example, a benzene ring,possibly substituted by one or two substituents chosen from among thealkyl and alkoxy groups having 1 to 10 C atoms and the halogen atoms; orseveral benzene rings, possibly substituted by one or more substituentschosen from among the alkyl and alkoxy groups having 1 to 10 C atoms andthe halogen atoms, for example from 2 to 4 rings, bonded to one anotherby a single bond or by a divalent group.

[0081] Said divalent group is chosen for example, from among:

[0082] a divalent group derived from a straight chain or branched alkylgroup (for example an alkylidene or alkylene group) with 1 to 10 Catoms, possibly substituted, preferably on the same carbon, by one ormore halogens chosen from among F, Cl, Br and I and/or by one or morehydroxyl groups and more preferably said divalent group is a divalentgroup derived from a perfluorinated alkyl group, for example aperfluorinated alkylene group.

[0083] a heteroatom chosen from among O, S,

[0084] where R₃ is chosen from among alkyl groups with from 1 to 10carbon atoms such as methyl, ethyl, isopropyl etc.

[0085] C₁ and C₂ can also each represent a condensed carbon polycyclicgroup, possibly substituted by one or more substituents chosen fromamong alkyl and alkoxy groups with 1 to 10 C atoms, and halogen atoms,comprising for example 2 to 5 benzene rings chosen for example fromamong naphthalene, phenanthrene, coronene, perylene etc.

[0086] C₁ and C₂ can also represent a heterocyclic group or a condensedheterocyclic group, with aromatic character such as thiophene, pyrazine,pyridine, furane, quinoline, quinoxaline, isobenzofurane, thisheterocyclic group possibly being substituted by one or moresubstituents chosen from among the alkyl groups (for example, methyl,ethyl, isopropyl, etc.) and alkoxy groups with from 1 to 10 C atoms andhalogen atoms (F, Cl, Br, I).

[0087] Among the polyimides that can be used in the context of theinvention, one may mention those in which C₁ is a benzene ring and C₂ anassembly of two benzene rings linked to one another by an oxygen bridge;or C₁ is made up of benzene rings, preferably two benzene rings linkedto one another by one or more perfluoroalkylene groups and C₂ is made upof benzene rings, preferably two benzene rings linked to one another byone or more divalent perfluoroalkyl or perfluoroalkylene groups; or C₁is a benzene ring and C₂ is a naphthalene ring; or C₁ and C₂ are bothnaphthalene rings.

[0088] Ar₁ and Ar₂ can be identical or different and each represent, forexample, a divalent benzene ring with meta or para links; possiblysubstituted by one or more substituents chosen from among the alkyl andalkoxy groups with from 1 to 10 C atoms, such as methyl, ethyl,isopropyl, butyl, methoxy etc. and halogen atoms; or several benzenerings, possibly substituted by one or more substituents chosen fromamong the alkyl and alkoxy groups with from 1 to 10 C atoms and thehalogen atoms, for example with 2 to 5 rings, linked to one another by asingle bond or by a divalent group.

[0089] Said divalent group is chosen for example from among

[0090] a divalent group derived from a linear or branched alkyl group(for example an alkylidene or alkylene group) with 1 to 10 C atoms,possibly substituted, preferably oh the same carbon atom by one or morehalogens, chosen from among F, Cl, Br and I and/or by one or morehydroxy groups. Preferably said divalent group is a divalent groupderived from a perfluorinated alkyl group, for example a perfluorinatedalkylene group.

[0091] a heteroatom chosen from among O, S,

[0092] where R₃ is chosen from among alkyl groups with from 1 to 10carbon atoms such as methyl, ethyl, isopropyl etc.

[0093] Ar₁ and Ar₂ can also each represent a condensed carbon polycyclicgroup possibly substituted by one or more substituents chosen from amongalkyl and alkoxy groups with 1 to 10 C atoms, and halogen atoms,comprising for example 2 to 5 benzene rings chosen for example fromamong naphthalene, phenanthrene, coronene, perylene etc.

[0094] Ar₁ and Ar₂ can also each represent a heterocyclic group or acondensed heterocyclic group, with aromatic character such as thiophene,pyrazine, pyridine, furane, quinoline, quinoxaline, isobenzofurane, thisheterocyclic group possibly being substituted by one or moresubstituents chosen from among the alkyl groups (for example, methyl,ethyl, isopropyl, etc.) and alkoxy groups with from 1 to 10 C atoms andhalogen atoms (F, Cl, Br, I).

[0095] According to the invention, at least one of the rings of Ar₂ forexample benzene rings or polyphenyl rings or others, is substitutedadditionally by one or more sulphonic acid groups.

[0096] The preferred polyimides are those in which Ar₁ is adiphenylmethane group and Ar₂ is a biphenyl-disulphonic group; where Ar₁is a benzene group, and Ar₂ is a biphenyl-disulphonic group; or Ar₁ is adiphenyl ether group and Ar₂ is a biphenyl-disulphonic group.

[0097] C₃ is, for example, a benzene ring or a naphthalene ring possiblysubstituted by one or more substituents chosen from among the alkyl andalkoxy groups with 1 to 10 C atoms and the halogen atoms.

[0098] Examples of groups C₁and C₂ are as follows:

[0099] Examples of Ar₁ groups are as follows

[0100] Examples of the Ar₂ groups are as follows:

[0101] Among the Ar₂ groups one may also mention any one of the Ar₁groups mentioned above carrying, in addition, one or more SO₃H groups onits ring(s) and/or heterocyclic ring(s).

[0102] Examples of C₃ groups are as follows:

[0103] The polyimides which are the subject of the invention can beobtained by any method known to a man skilled in the art for thepreparation of polyimides in general.

[0104] Examples of known methods of preparation of polyimides arenotably the following:

[0105] reaction of a di-anhydride and a di-amine

[0106] reaction of a di-acid di-ester and a di-amine.

[0107] It is obvious that the polyimides according to the invention canbe prepared by methods which derive from the methods mentioned above orby other methods that can be used for the synthesis of polyimides.

[0108] The required modifications and optimisations of methods known anddescribed in the literature can be easily carried out by a man skilledin the art.

[0109] Preferably, to prepare the polyimides according to the invention,the condensation of di-anhydrides onto di-amines in a two stagesynthesis is used.

[0110] Such a method is currently used on an industrial scale and onlyrequires small adaptations to allow the preparation of the polyimidesaccording to the invention.

[0111] The synthesis of a condensation polyimide corresponds generallyto the following diagram and is carried out in two stages:

[0112] In the first stage, the condensation reaction of a di-anhydridewith a di-amine is carried out in order to obtain a polyamide-acidintermediate of formula (VI)called a “prepolymer” according to thediagram below given for the first type of repeating structure forpolyimides of the invention.

[0113] or according to the diagram below for the second type ofrepeating structure for polyimides according to the invention:

[0114] The starting products which are the di-anhydrides (II) and (II′)or the bi-primary di-amines (III) (IV) are products which are readilyavailable and, for the most part, of low cost.

[0115] Because of this, and conforming to one of the particularlyinteresting characteristics of this patent application, the polymersprepared and consequently the membranes obtained from these polymershave a relatively low cost: lowered by a factor of the order of 10,which is clearly a lower cost than the membranes of the prior artcurrently used notably in fuel cells.

[0116] One can also envisage a concomitant reduction in the cost priceof fuel cells which would open up for them applications in fields suchas providing energy for automobiles which until now they have been along way from providing because of their excessive cost.

[0117] Among the di-anhydrides of general formula (II)

[0118] where C₁ has the meaning given above.

[0119] By way of examples, one may mention the di-anhydrides of thefollowing aromatic tetracarboxylic acids: benzene1,2,3,4-tetracarboxylic acid, benzene 1,2,4,5-tetracarboxylic acid,1,1′-biphenyl 2,3′,5′,6′-tetracarboxylic acid, 1,1′-biphenyl3,3′,4,4′-tetracarboxylic acid, 1,1′-biphenyl 2,2′,3,3′-tetracarboxylicacid, 1,1′,1″-terphenyl 2,3′,5′,6′-tetracarboxylic acid, naphthalene1,2,5,6 tetracarboxylic acid, naphthalene 2,3,6,7 tetracarboxylic acid,naphthalene 1,2,4,5 tetracarboxylic acid, naphthalene 1,2,5,6tetracarboxylic acid, naphthalene 1,4,5,8 tetracarboxylic acid, perylene3,4,9,10 tetracarboxylic acid, phenanthrene 1,8,9,10 tetracarboxylicacid, 4,4′-oxybis-(benzene 1,2-dicarboxylic) acid, 4,4′-thiobis-(benzene1,2-dicarboxylic) acid, 4,4′-sulphonylbis-(benzene 1,2-dicarboxylic)acid, 4,4′-methylenebis-(benzene 1,2-dicarboxylic) acid,4,4′-difluoromethylenebis-(benzene 1,2-dicarboxylic) acid,3,3′-carbonylbis-(benzene 1,2-dicarboxylic) acid,4,4′-carbonylbis-(benzene 1,2-dicarboxylic) acid, 4,4′-methyl-1ethylidene-1,1-bis (benzene 1,2-dicarboxylic) acid,4,4′-trifluoromethyl-1 trifluoro-2,2,2 ethylidene-1,1-bis(benzene1,2-dicarboxylic) acid, 4,4′-phenylene-1,3-bis (carbonylbenzene1,2-dicarboxylic) acid, 4,4′-phenylene-1,3-bis (carbonylbenzene1,2-dicarboxylic) acid, 4,4′-phenylene-1,4-bis (carbonylbenzene1,2-dicarboxylic) acid, 4,4′-phenylene-1,3-bis (oxybenzene1,2-dicarboxylic) acid, 4,4′-phenylene-1,4-bis (oxybenzene1,2-dicarboxylic) acid, 4,4′-methyl-1 ethylidene-1,1-bis(phenylene-1,4-oxy)-bis (benzene 1,2-dicarboxylic) acid, pyrazine2,3,5,6-tetracarboxylic acid, thiophene 2,3,4,5-tetracarboxylic acid and3,3′,4,4′-tetracarboxy benzanilide.

[0120] The di-anhydrides of general formula (II′)

[0121] where C₂ which has the meaning already given to it above can bechosen from among the same compounds mentioned above for thedi-anhydrides of formula (II).

[0122] Among the biprimary di-amines of formula (III) H₂N—Ar₁—NH₂ whereAr₁ has the same meaning already given to it above and which can be usedin the preparation of polyimides of the invention, one may mention1,3-diaminobenzene, 1,4-diaminobenzene, 6-methyl-1,3-diaminobenzene,2-methyl-1,3-diaminobenzene, 5-methyl-1,3-diaminobenzene,4,4′-diamino-1,1′-biphenyl, 3,3′-4,4′-diamino-dimethyl-1,1′-biphenyl,4,4′-diamino-3,3′-dimethoxy-1,1′-biphenyl,4,4′-diamino-3,3′-dichloro-1,1′-biphenyl, methylenebis(4,4′-benzeneamine), methylenebis(3,3′-benzene amine) methylenebis(3-methyl-4,4′-benzene amine), methylenebis (3-isopropyl-4,4′-benzeneamine), oxybis(4,4′-benzene amine), oxybis(3,3′-benzene amine),carbonylbis(4,4′-benzene amine), carbonylbis(3,3′-benzene amine),thiobis(4,4′-benzene amine), thiobis(3,3′-benzene amine),sulphonylbis(4,4′-benzene amine), sulphonylbis(3,3′-benzene amine),hydroxymethylenebis(4,4′-benzene amine),hydroxymethylenebis(3,3′-benzene amine),difluoromethylenebis(4,4′-benzene amine),1-methylethylidenebis(4,4′-benzene amine),1-trifluoromethyl-2,2,2-trifluoroethylidenebis(4,4′-benzene amine),1,3-dioxyphenylenebis(3,3′-benzene amine),1,3-dioxyphenylenebis(4,4′-benzene amine),1,4-dioxyphenylenebis(3,3′-benzene amine),1,4-dioxyphenylenebis(4,4′-benzene amine), 3,3′-diamino-benzanilide,3,4′-diamino-benzanilide, 3′,4-diamino-benzanilide,4,4′-diamino-benzanilide, bis(3-aminophenyl) dimethylsilane,bis(4-aminophenyl) dimethylsilane and 9-fluoro-9-ylidene bisphenylamine.

[0123] Among the sulphonated biprimary diamines of formula (IV)

[0124] where Ar₂ has the meaning already given to it above and which canbe used in the context of the preparation of polyimides of theinvention. One may mention for example 1,4-diaminobenzene-3-sulphonicacid, 4,4′-diamino-1,1′-biphenyl-2,2′-disulphonic acid.

[0125] The condensation of the di-anhydrides with the diamines canoccur, within the context of the invention with or without a chainlimiting agent.

[0126] However, it is preferable to use a chain limiting agent,preferably of the anhydride type, since one thereby avoids the presenceon the end of the chain of an amine group which can be easily oxidised.

[0127] Among the chain limiting agents of the anhydride type that aresuitable, for the preparation of polyimides according to the invention,one may mention functional anhydrides with the following formula (V):

[0128] where C₃ has the meaning already-given to it.

[0129] Examples of anhydrides of formula (V) are phthalic anhydride,3-fluorophthalic anhydride and naphthalene-1,8-dicarboxylic acidanhydride.

[0130] In a second stage, the synthesis of the polyimide itself iscarried out according to the following diagram given by way of examplefor the first type of repeating structure;

[0131] In the first step of the method for the preparation of polyimidesaccording to the invention, the starting reactants can be dissolved in asuitable solvent.

[0132] The solvent can be any suitable solvent known to a man skilled inthe art appropriate to the polycondensation reaction of a di-anhydridewith a diamine.

[0133] In a preferred embodiment, the solvent is an aprotic polarsolvent chosen, for example, from among dimethylformamide,dimethylacetamide, N-methylpyrrolidone, alone or in a mixture with, forexample, aromatic solvents such as xylene or solvents of the glycolether type.

[0134] The solvent can also be a solvent of the phenolic type, that isto say, it is chosen, for example, from among phenol, phenolssubstituted by one or more halogens (Cl, I, Br, F), the cresols (o-, m-,and p-cresol), cresols substituted by a halogen (Cl, I, Br, F) andmixtures of these.

[0135] The preferred solvents will be constituted by m-cresol and amixture of para-chlorophenol or meta-chlorophenol and phenol, forexample in the proportion of 20% phenol to 80% of para- or meta-chlorophenol.

[0136] By reactants, one understands the compounds (II), (III), (II′),(IV) and possibly (V) already described above. One can, for example,begin by reacting, as initial reactants present, the mixture of adi-anhydride (II) and/or a di-anhydride (II′) with a sulphonated diamine(IV) and then add afterwards the second diamine (III).

[0137] All the combinations possible relating to the order of additionof the reactants can easily be determined by a man skilled in the art.

[0138] The condensation reaction of the first step is carried out in thesolvent generally at ordinary temperature, for example 20-25° C. and thepolyamide-acid intermediate or prepolymer is formed.

[0139] The polyamide-acid intermediate obtained can be used, forexample, to prepare a film, for example by casting; the solvent is thenevaporated at a temperature of from 50 to 150° C. to give a finalpolyamide acid and the polyimide according to the invention is obtainedconforming to the second step either by heat treatment at a temperaturebelow 250° C. (that is to say below the desulphonation temperature) orby chemical dehydration by using acetic anhydride.

[0140] Another preferred alternative consists of heating the startingsolution including, in the first case all of the reactants, to atemperature, for example, of from 120 to 200° C. for a period of, forexample, from 6 to 72 hours.

[0141] The starting solution can, as has already been mentioned above,only contain, in the second case, a part of the reactants necessary forthe preparation of the final polyimide, for example, the startingmixture may only include compounds (II) and/or (II′) and (IV). In thiscase, the mixture is brought to a temperature, for example, of from 120to 200° C. for a period of, for example, of from 6 to 72 hours and thenthe mixture is allowed to cool to a temperature, for example, of 20 to50° C. and the rest of the reactants, for example, compounds (III) and(II) are added.

[0142] The one brings the temperature of the mixture to, for example,between 120 and 200° C. for a period of from 6 to 72 hours.

[0143] In both cases, the heating causes cyclisation of the amide to animide and the final product, the polyimide of the invention is obtained.

[0144] The polymerisation is stopped by cooling the solution, forexample to ambient temperature. Then the solution is poured, preferablyslowly, under strong stirring, into a receptacle containing, for examplemethanol or ethanol.

[0145] The generally fibrous solid that precipitates is separated, forexample, by filtration and is then preferably washed one or more times,for example, with a sufficient quantity of, for example, methanol.

[0146] The polymer obtained is then dried, preferably at a temperatureof from 50 to 120° C., for example in a force ventilated oven for asufficient period of time.

[0147] A further subject of this invention is a film or a membrane,comprising the sulphonated polyimide described above.

[0148] The films or membranes can be prepared in the traditional manner,for example, by casting, that is to say the polymer according to theinvention is dissolved in a suitable solvent such as cresol or phenol,and then poured onto a flat surface such as a glass plate and then driedto form a film with a thickness of from 5 to 200 μm.

[0149] The films can be used to prepare membranes that isolate, inparticular, the anode and cathode compartments of a fuel cell that canoperate, for example, with the following systems:

[0150] hydrogen, alcohols such as methanol, at the anode

[0151] oxygen, or air at the cathode.

[0152] Another subject of this invention is a fuel cell devicecomprising one or more membranes comprising the sulphonated polyimideaccording to the invention.

[0153] Because of its excellent mechanical properties, the membrane canbe subjected to the stresses (clamping etc.) linked to it being mountedin such a device, without damage.

[0154] The fuel cell can correspond to the diagram already given in FIG.1, that is to say, the membrane according to the invention is positionedat (1) between two electrodes (11) for example made of carbon fabricthat has been treated with platinum (or any other noble metal),preferably impregnated with a compound such as Nafion® or the polyimideitself, with the aim of having one electrode per unit volume.

[0155] This assembly can then be arranged, for example, between twoplates made of sealed graphite, impregnated with resins, which, on theone hand ensure the distribution of the hydrogen (4) or other compound,on the anode side (7) and on the other hand, of oxygen (5) on thecathode side (8).

[0156] The cell can also comprise means (not shown) of adjusting thetemperature, such as copper plates thermostated by heating rods in amonocell or heat exchangers in a module, and means of regulating theoperation of the cell which are connected to the external circuit (10);the means are constituted, for example, by flow regulators, temperatureregulators, pressure regulators and an electronic load to regulate thecurrent. The temperature of the cell is generally maintained between 50and 80° C. and under these conditions, it produces, for example, acurrent of 0.25 A/cm² with a voltage of 0.6 V and this over a very longduration that can reach as much as 3000 hours, which demonstrates theexcellent properties of thermal stability and others of the membrane andits excellent electrical properties in comparison with the membranes ofthe prior art, for example made of Nafion® (0.25 A/cm²; 0.7 V; 5000hours).

[0157] The invention will now be described, making reference to thefollowing examples, given as examples and being non-limitative.

EXAMPLE 1

[0158] The polycondensation reaction is carried out in a 500 cm³ glassreactor fitted with an anchor stirring system, an inlet for an inertgas, such as nitrogen and a temperature probe. A thermostated oil bathis used to adjust the temperature of the reaction medium.

[0159] The reactor is charged with 250 g of m-cresol and 10 g (0.029mole) of 4,4′-diamino-(1,1′-biphenyl)-2,2′-disulphonic acid. Then 30 g(0.096 mole) of 5,5′-oxy-bis-(1,3-iso-benzofurandione) is added.

[0160] This mixture is brought to a temperature of 180° C. for 4 hours.During this period, the viscosity of the medium increases progressively.

[0161] Then, the heating is stopped whilst maintaining the agitation andthe reaction mixture is allowed to cool to a temperature of about 50° C.

[0162] Then, 13.4 g (0.067 mole) of 4,4′-methylene bis-benzene amine allat once before bringing the temperature of the mixture once again to180° C. for about two hours, a period during which the viscosityincreases rapidly.

[0163] The polymerisation is stopped by cooling the solution to ambienttemperature, and then by pouring it slowly, under strong agitation intoa receptacle containing two liters of methanol.

[0164] The fibrous solid which precipitates is isolated by filtrationand washed twice with two liters of methanol.

[0165] The polymer thus obtained is then dried in a force ventilatedoven at 120° C. for about six hours.

[0166] Its ion exchange capacity is 1.15 meq/g.

EXAMPLE 2

[0167] The method of Example 1 is used to prepare a sulphonatedpolyimide by reacting 220 g of m-cresol, 33.5 g (0.108 mole) of5,5′-oxy-bis (1,3-isobenzofurandione), 7.5 g (0.021 mole) 4,4′diamino-(1,1′-biphenyl)-2,2′-disulphonic acid and 17.25 g of4,4′-methylene bis-benzene amine.

[0168] The polymer obtained had an ion exchange capacity of 0.8 meq/g.

EXAMPLE 3

[0169] The method of Example 1 is used to prepare a sulphonatedpolyimide by reacting 300 g of m-cresol as solvent, 45 g (0.145 mole) of5,5′-oxy-bis-(1,3-isobenzofurandione), 16.2 g (0.043 mole) of4,4′-diamino-5,5′-dimethyl-(1,1′-biphenyl)-2,2′-disulphonic acid and20.25 g (0.102 mole) of 4,4′-methylene-bis-benzene amine.

[0170] The polymer thus obtained has an ion exchange capacity of 1.14meq/g.

EXAMPLE 4

[0171] The equipment for the reaction described in Example 1 is used forthe preparation of a sulphonated polyimide.

[0172] The reactor is charged with a mixture of solvents made up of 210g of phenol and 140 g of para-chlorophenol.

[0173] Then, all at once, 5 g (0.015 mole) of4,4′-diamino-(1,1′-biphenyl)-2,2′-disulphonic acid, 12.97 g (0.048 mole)of the di-anhydride of 1,4,5,8-naphthalene tetracarboxylic acid and 6.7g (0.034 mole) of 4,4′-oxy-benzene amine.

[0174] The reaction mixture is heated to 150° C. for five hours.

[0175] After cooling to 60° C., the solution is poured, under strongagitation, into two liters of methanol.

[0176] The fibrous solid which forms is isolated by filtration andtreated twice with 500 cm³ of methanol.

[0177] The polymer thus obtained is dried under vacuum at 120° C. forsix hours.

[0178] Its ion exchange capacity is 1.28 meq/g.

EXAMPLE 5

[0179] The method of Example 4 is used to prepare a sulphonatedpolyimide by reacting, in a mixture of solvents made up of 180 g ofphenol and 140 g of para-chlorophenol, 8.32 g (0.0242 mole) of4,4′-diamino-(1,1′-biphenyl)-2,2′-disulphonic acid, 12.975 g (0.048mole) of the di-anhydride of 1,4,5,8-naphthalene-teracarboxylic acid and8.08 g (0.024 mole) of4,4′-(2,2,2-tifluoro-(1-trifluoromethyl)-ethylidene)-benzene amine.

[0180] The polymer obtained has an ion exchange capacity of 1.73 meq/g.

EXAMPLE 6

[0181] This example illustrates how the films of sulphonated polyimideaccording to this invention can be used, to prepare membranes thatisolate the anode and cathode compartments of a fuel cell able tooperate with hydrogen and oxygen at the cathode.

[0182] A membrane of sulphonated polyimide according to the invention(prepared from the polymer of Example 1) with an ion exchange capacityof 1.15 meq/g and of thickness 80 μm, is positioned between twoelectrodes made of platinised carbon fabric (0.35 mg of Pt/cm²) andimpregnated with Nafion® (0.8 mg/cm²).

[0183] This assembly is then inserted between two graphite plates which,on the one hand, ensure the distribution of the hydrogen on the anodeside and of oxygen on the cathode side, and on the other hand providethe electrical conductivity.

[0184] The system is sealed by the use of flat gaskets made of “Viton”elastomer (not shown). For heating, for example, two copper plates areused.

[0185] An electronic load allows the operation of the cell to beregulated with respect to voltage or current. The experiment is carriedout at a temperature of 50° C. with the pressure of oxygen and ofhydrogen equal to 4 bars.

[0186] The membrane of phthalic type sulphonated polyimide according tothe invention allowed a current of 500 mA/cm² to be obtained at avoltage of between 300 and 400 mV.

EXAMPLE 7 (COMPARATIVE)

[0187] A membrane made of Nafion® 117, a polymer having the followingstructure

[0188] is used as a reference and the membrane is tested under -the sameconditions as those defined in Example 6 for the membrane of phthalictype sulphonated polyimide according to the invention.

[0189]FIG. 2 shows the polarisation curves obtained, on the one handwith the membrane according to the invention (full line) and on theother hand with the reference membrane made of Nafion®117 (broken line).They provide evidence of the equivalent and indeed superior performancethat is obtained with the membrane according to the invention.

EXAMPLE 8

[0190] A membrane of phthalic type sulphonated polyimide (the polymerfrom Example 1) with an ion exchange capacity of 1.15 meq/g, is fittedin a cell in accordance with Example 6.

[0191] The cell is operated at a temperature held at 50° C. and a stablecurrent of 150 mA/cm² is measured for a voltage of 400 mV for more than400 hours.

EXAMPLE 9

[0192] A membrane of phthalic type sulphonated polyimide (the polymerfrom Example 1) with an ion exchange capacity of 1.15 meq/g, is fittedin a cell in accordance with Example 6.

[0193] The cell is operated at a temperature held at 70° C. and avoltage of 600 mV is measured for a current of 200 mA/cm² whichdecreases progressively by 1.2 mV per hour.

EXAMPLE 10

[0194] A membrane of naphthalenic type sulphonated polyimide (thepolymer from Example 4), of thickness 170μm, having an ion exchangecapacity of 1.28 meq/g, is placed in a cell in accordance with Example6.

[0195] The cell is operated at a temperature held at 70° C. and acurrent of 1 A/cm² is measured with a voltage of 0.4 V.

EXAMPLE 11 (COMPARATIVE)

[0196] A membrane made of Nafion®117 is positioned in the same cell andis tested under the same conditions of operation as those defined forExample 10 for the membrane of naphthalenic type sulphonated polyimideaccording to the invention.

[0197]FIG. 3 which shows the polarisation curves obtained, on the onehand with the membrane according to the invention (full line) and on theother hand with the reference membrane made of Nafion®117 (broken line)provides evidence of the equivalent and indeed superior performance thatis obtained with the membrane according to the invention.

EXAMPLE 12

[0198] A membrane of the fluorinated naphthalenic type sulphonatedpolyimide (the polymer from Example 5), the thickness of which is 150μmis positioned in a cell in accordance with Example 6. The cell isoperated at a temperature held at 70° C. and a voltage of 650 mV ismeasured for a current of 250 mA/cm² which remains stable for more than2500 hours.

[0199] This example demonstrates once again the superior durabilityproperties of polymers according to the invention, namely that they areable to achieve several thousand hours of operation, which is compatiblewith an application, for example, in an electric vehicle where adurability of around 3000 hours is demanded.

1. A sulphonated polymer characterised in that it comprises repeatingstructures of formula (In)

and repeating structures of formula (I_(m))

in which the groups C₁ and C₂ can be identical or different and eachrepresent a tetravalent group that includes at least aromatic carbonring possibly substituted and having from 6 to 10 atoms of carbon and/ora heterocyclic ring with aromatic character, possibly substituted andhaving from 5 to 10 atoms and including one or more heteroatoms chosenfrom among S, N, and O; C₁ and C₂ each forming with the neighbouringimide groups rings with 5 or 6 atoms. the Ar₁ and Ar₂ groups can beidentical or different and each represent a divalent group that includesat least one aromatic carbon ring possibly substituted and having from 6to 10 atoms of carbon and/or a heterocyclic ring with aromaticcharacter, possibly substituted and having from 5 to 10 atoms andincluding one or more heteroatoms chosen from among S, N and O; at leastone of said aromatic carbon rings and/or heterocyclic rings of Ar₂being, in addition substituted by at least one sulphonic acid group. 2.A sulphonated polyimide according to claim 1 , characterised in that itcorresponds to the following general formula (I):

in which C₁, C₂, Ar₁ and Ar₂ have the meanings already given to themabove and where each of the groups R, and R₂ represent NH₂ or a group offormula

where C₃ is a divalent group that includes at least one aromatic carbonring possibly substituted and having from 6 to 10 carbon atoms and/or aheterocyclic ring with aromatic character, possibly substituted andhaving from 5 to 10 atoms and including one or more heteroatoms chosenfrom among S, N and O. C₃ forming with the neighbouring imide group aring with 5 or 6 atoms.
 3. A polyimide according to claim 2 ,characterised in that m represents a whole number from 1 to 20, nrepresents a whole number from 1 to 30 and o represents a whole numberfrom 1 to
 10. 4. A polyimide according to claim 2 , characterised inthat the numbers m and n are chosen in such a way that the equivalentmolecular weight, defined as the weight of the polymer in grams perequivalent of sulphonic acid, is from 500 to
 2500. 5. A polyimideaccording to claim 1 or claim 2 , characterised in that its molecularweight is from 10,000 to 100,000.
 6. A polyimide according to claim 1 orclaim 2 , characterised in that in the formulae (In), (Im) and (I), C₁and C₂ can be identical or different and each represent a benzene ring,possibly substituted by one or two substituents chosen from among alkyland alkoxy groups with 1 to 10 C atoms and halogen atoms; or severalbenzene rings, possibly substituted by one or more substituents chosenfrom among alkyl and alkoxy groups with 1 to 10 C atoms and halogenatoms, linked to one another by a single bond or by a divalent group. C₁and C₂ can also each represent a condensed carbon polycyclic group,possibly substituted by one or more substituents chosen from among alkyland alkoxy groups with 1 to 10 C atoms, and halogen atoms, C₁ and C₂ canalso represent a heterocyclic group or a condensed heterocyclic group,with aromatic character, this heterocyclic group possibly beingsubstituted by one or more substituents chosen from among the alkyl andalkoxy groups with from 1 to 10 C atoms and the halogen atoms. Ar₁ andAr₂ can be identical or different and each represent, for example, adivalent benzene ring with meta or para links; possibly substituted byone or more substituents chosen from among the alkyl and alkoxy groupswith from 1 to 10 C atoms and the halogen atoms; or several benzenerings, possibly substituted by one or more substituents chosen fromamong the alkyl and alkoxy groups with from 1 to 10 C atoms and thehalogen atoms, linked to one another by a single bond or by a divalentgroup. Ar₁ and Ar₂ can also each represent a condensed carbon polycyclicgroup possibly substituted by one or more substituents chosen from amongalkyl and alkoxy groups with 1 to 10 C atoms, and the halogen atoms, Ar₁and Ar₂ can also each represent a heterocyclic group or a condensedheterocyclic group, with aromatic character, this heterocyclic grouppossibly being substituted by one or more substituents chosen from amongthe alkyl and alkoxy groups with from 1 to 10 C atoms and halogen atoms.7. A polyimide according to claim 2 , characterised in that C₃ is abenzene or naphthalene ring possibly substituted by one or moresubstituents chosen from among alkyl and alkoxy groups with 1 to 10 Catoms, and the halogen atoms.
 8. A polyimide according to claim 6 ,characterised in that said divalent group is chosen from among adivalent group derived from a straight chain or branched chain alkylgroup with 1 to 10 C atoms, possibly substituted, by one or morehalogens chosen from among F, Cl, Br and I and/or by one or morehydroxyl groups. a heteroatom chosen from among O, S,

where R₃ is chosen from among alkyl groups with from 1 to 10 carbonatoms.
 9. A polyimide according to claim 6 , characterised in that C₁ isa benzene ring and C₂ is a group of two benzene rings linked to oneanother by an oxygen bridge.
 10. A polyimide according to claim 6 ,characterised in that C₁ is made up of benzene rings linked by one ormore perfluoroalkylene groups and C₂ is made up of benzene rings linkedby one or more divalent perfluoroalkyl groups or perfluoroalkylenegroups.
 11. A polyimide according to claim 6 , characterised in that C₁is a benzene ring and C₂ is a naphthalene ring.
 12. A polyimideaccording to claim 6 , characterised in that C₁ and C₂ are bothnaphthalene rings.
 13. A polyimide according to claim 6 , characterisedin that Ar₁ is a diphenylmethane group and Ar₂ is a biphenyl disulphonicacid group.
 14. A polyimide according to claim 6 , characterised in thatAr₁ is a benzene group and Ar₂ is a biphenyl disulphonic acid group. 15.A polyimide according to claim 6 , characterised in that Ar₁ is adiphenyl ether group and Ar₂ is a biphenyl disulphonic acid group.
 16. Amembrane comprising a polyimide according to claim 1 or claim 2 .
 17. Afuel cell device comprising at least one membrane according to claim 16.